Automated fiber placement is a process that is widely used for fabricating composite structures from pre-impregnated composite materials, such as carbon fiber, fiber glass and Kevlar. The materials, for use in automated fiber placement, typically take the form of a strip or yarn of composite material impregnated with a resin, with such strips or yarns being commonly referred to as tapes or tows, those terms being used in a generally interchangeable manner herein.
During automated fiber placement, groups of tows or tapes are deposited on a mold or tool by an automated fiber placement machine, to form a composite structure. The fiber placement machine typically includes a computer controlled, robotic, fiber placement head which has provisions for simultaneously handling groups of, for example, 12, 24, or 32 tows, which are positioned substantially parallel to one another by the fiber placement head to form a substantially contiguous band, of pre-impregnated composite material. Often, one or more bands of material form a layer or ply of material, having the tows in the ply oriented substantially parallel to one another in the ply. Successive plys may be laid on top of a preceding ply, with the tows in the successive plys being oriented in a different direction than adjacent plys, to create a completed part having desired structural capabilities.
During the fiber placement process, it is sometimes necessary to cut and stop the feed of individual tows, thus removing them from the band of material, in order to reduce the width of the band so that it may be placed onto the surface of the mold or tool in a manner that precludes having excessive gaps between successive bands of material, or having the edges of successive bands of material unintentionally overlap one another. In similar fashion, it is often desirable to add tows to the band in order to increase its width, at various stages of the automated fiber placement process, in order to facilitate manufacture of the composite structure. The process of removing or adding tows is commonly referred to “cut and add”. On large structures, it may also be necessary to periodically splice in fresh tows during fabrication, from replacement reels of material, as the material on the original supply reel is exhausted.
Automated fiber placement machines are capable of depositing material onto a tool surface at high feed-rates, of, for example, 1200 inches/minute or higher. For maximizing productivity, it is desirable to operate an automated fiber placement machine at such high feed-rates throughout the fabrication of a composite structure. It is highly desirable, therefore, that automated fiber placement machines be capable of modifying the width of the band of material being applied without stopping, or slowing down, the machine to cut or add tows to the material band. In the vernacular of the automated fiber placement industry, it is highly desirable that automated fiber placement machines be capable of cutting or adding tows “on-the-fly.”
From the foregoing description, it will be appreciated that fabrication of a composite structure by automated fiber placement is a highly complex process, requiring considerable upfront effort during design of the structure, associated production tooling, and in setting up the automated fiber placement machine, to ensure that each and every tow of material is properly placed during the automated fiber placement process, in a manner that will result in a structure having a desired geometry and structural properties. Due to the complexity involved in both the design and production of the composite structure by automated fiber placement, it is common practice to utilize computerized tools for both designing the structure, and in programming the automated fiber placement machine to properly move and operate the robotic fiber placement head, feed out material, cut and add tows, and in some cases to also move the tooling in synchronization with the robotic head during automated lay-up of the composite structure on the tooling. The process of designing a composite structure for automated fabrication is discussed in U.S. Pat. No. 6,799,081 B1, to Hale et al.
Even after the composite structure, the tooling, and the automated fiber placement machine are designed and set up to provide for proper placement of the tows during fabrication, problems inherent in the production of composite structures make it necessary that the resulting composite structure be closely inspected to ensure that each and every tow was indeed properly placed during fabrication. It is necessary, for example, to ensure that tows and/or bands of tows were properly cut and/or added at a desired location during fabrication of the structure. It is also necessary to detect any improper placement, and/or other anomalous conditions, such as fiber twists, excessive resin build-up, “fuzz balls,” bad sections of tow, foreign matter and/or unintended gaps or overlapping of the tows.
Although it is highly desirable to perform such inspection on-the-fly in real-time, during automated fiber placement, factors such as the high speed at which the automated fiber placement takes place, complex shaped and large sized composite structures, and rapid movement of the robotic fiber placement head makes such real-time inspection very difficult. In the past, the difficulty involved has, substantially, precluded real-time, on-the-fly, inspection, and required reliance on methods which could only be carried out after the structure is completely fabricated, or requiring that the automated fiber placement process be stopped for a period of time to allow inspection by visual, sonic, magnetic resonance imaging, or x-ray to determine the location of the anomalies.
It is also desirable that the inspection be carried out in real-time, and that the results of the inspection be available substantially in real-time, while the composite structure is being fabricated, so that any anomalies or other problems discovered during inspection may be repaired, or otherwise dealt with, prior to completion of fabrication of the composite structure.
In one prior attempt at performing such real-time inspection, disclosed in U.S. Pat. No. 5,562,788 to Kitson et al., a visual imaging system utilizes a laser analog displacement sensor to detect the edges of individual composite tows, and utilizes the location of the edges of the individual tows, to compute the location and size of gaps between the individual tows. The location and size of the computed gaps between the tows is then utilized as an indicator for flaws, such as excessive gaps, overlaps, twisted tows, or the presence of foreign material under the tow. Because Kitson '788 relies solely upon computations based on the sensed edges of the individual tows, only an approximation of any anomalous condition is provided. Conditions such as fiber twists, excessive resin buildup, or fuzz balls, for example, are not directly detected by the methods and apparatus of Kitson. The methods and apparatus of Kitson also require that massive amounts of data be collected, stored, and analyzed.
All of the challenges and problems discussed above are exacerbated in structures, such as aircraft flight surfaces and entire fuselage sections, of the type being utilized in modern military and commercial aircraft. Due to their large size and complexity, and the necessity for providing substantially infallible structural integrity under high loading conditions, it is highly desirable that inspection processes be carried out in real-time, during fabrication of the composite structure by automated fiber placement, so that any anomalous conditions can be identified and potentially rectified prior to continuing with the fabrication process. Due to the complex nature of the three dimensional placement of the tows, and multi-axis motions of the tooling and robotic fiber placement head during fabrication, it is also highly desirable that a method and apparatus be provided for indicating the location of any detected anomalies, and/or properly positioning the composite structure to allow any additional inspection or corrective action to be conveniently made once an anomalous condition is detected. Prior inspection systems, such as the one disclosed by Kitson '788 do not provide for such convenient additional in-process inspection and corrective action.
In order to address the unique problems of fabricating very large composite structures, such as aircraft control surfaces and fuselages, for example, the Assignee of the present invention has developed a number of advanced capabilities, including the use of multiple robotically controlled fiber placement heads, and automatically replaceable creels for holding spools of the pre-impregnated composite material. These and other advanced methods and apparatuses for performing automated fiber placement are disclosed in a number of the Assignee's related published U.S. patent applications, such as: 2006/0070697 A1, to Hoffmann, titled, METHOD AND APPARATUS FOR DIRECTING RESIN-IMPREGNATED TAPE; 2005/0247396 A1, to Oldani et al., titled, AUTOMATED FIBER PLACEMENT USING MULTIPLE PLACEMENT HEADS, REPLACEABLE CREELS, AND REPLACEABLE PLACEMENT HEADS; 2005/0236735 A1, to Oldani et al., titled, FORMING A COMPOSITE STRUCTURE BY FILAMENT PLACEMENT ON A TOOL SURFACE OF A TABLET; 2005/0269016 A1, to Oldani et al., titled, AUTOMATED FORMING OF A PRE-IMPREGNATED COMPOSITE STRUCTURAL ELEMENTS; and 2005/0240291 A1, to Oldani et al., titled PERFORMING HIGH-SPEED EVENTS “ON-THE-FLY” DURING FABRICATION OF A COMPOSITE STRUCTURE BY AUTOMATED FIBER PLACEMENT.
Prior inspection methods and apparatuses, including Kitson '788, do not address the additional complexity and unique problems involved in performing inspection where advanced, state-of-the-art, methods and apparatuses, of the type described in the Assignee's patent applications listed above are utilized.
What is needed, therefore, is an improved method and apparatus for performing high-speed inspection, preferably on-the-fly, in real time, during fabrication of a composite structure by automated fiber placement. It is also desirable that such an improved apparatus and method include provisions for indicating the location of any anomalies during fabrication, to allow for additional inspection and possible in-process correction. It is further desirable that such improved methods and apparatuses provide for post processing to record one or more of: proper and/or improper placement of the fibers; and/or anomalies detected. It is yet further desirable that an improved apparatus and/or method provide images displaying some or all anomalous conditions detected.